The present invention relates to a thermal transfer image receiving sheet, more particularly to a thermal transfer image receiving sheet capable of forming an image of high density and high resolution.
Various thermal transfer methods have been heretofore known. Of these, there has been proposed a method in which a sublimable dye is used as a recording agent and is supported on a substrate sheet such as a paper or a plastic film to prepare a thermal transfer sheet, and using the thermal transfer sheet, various full color images are formed on a thermal transfer image receiving sheet which is capable of being deposited with a sublimable dye, for example, a thermal image receiving sheet having a dye receptor layer on a paper or a plastic film.
In such a case, a thermal head of a printer is used as a heating means, so that a great number of color dots of three or four colors are transferred onto the thermal transfer image receiving sheet under heating for a short period of time, thereby to reproduce a full color image of an original. Such images as obtained above are very sharp because the used colorant is a dye, and are also excellent in transparency. Therefore, the images are excellent in half tone reproducibility and gradation properties, and are substantially the same as those formed by the conventional offset printing and gravure printing. Further, when the above image forming method is used, there can be formed images having high quality which are comparable to full color photographic images.
As the substrate sheet of the thermal transfer image receiving sheet used in the above sublimation type thermal transfer method, a plastic sheet, a laminate sheet of a plastic sheet and a paper, a synthetic paper, etc. are employed. However, in order to widely utilize the sublimation type thermal transfer method also in common offices, it is required to use ordinary papers such as a coat paper (i.e., art paper), a cast coat paper and a PPC paper as the substrate sheet of the image receiving sheet. In the case where such ordinary office papers are used as the substrate sheet and a dye receptor layer is formed thereon, there resides such a problem that when the paper is coated with an aqueous solution of a water-soluble resin or an aqueous emulsion of a water-insoluble resin to fill up the paper surface, water content is absorbed by a coat layer or a cast coat layer of the paper, resulting in waviness of the paper substrate in the drying procedure after the coating procedure. If the paper is coated with a solution of a hydrophobic resin, such problem hardly occur, but in this case other problems reside. That is, when a large amount of the solution is used in order to enhance the printed image quality, marked curling is brought about with moisture variation, because the pulp paper substrate has moisture absorption characteristics and the receptor layer is hydrophobic, etc., resulting in deterioration of printed image quality. Moreover, rubbing with a conveying roller during the printing procedure causes occurrence of paper powder.
Further, when the above-mentioned thermal transfer method is carried out, especially when an image having high gradation characteristics and shades of large difference is demanded, a large heat energy is out put from the thermal head within an area of high density, and thereby various problems occur. For example, the surface of the receptor layer suffers depressed and protruded portions, the substrate sheet of the thermal transfer image receiving sheet suffers thermal deformation in the excessive case, and curling is brought about on the thermal transfer image receiving sheet, whereby quality of the obtained image deteriorates. In the case of forming a full color image, printing procedures of 3 to 4 times are conducted on the same region of the receptor layer. Therefore, if the surface of the receptor layer is depressed and protruded, the transference of the dye in the second or the subsequent transferring stages is made ununiformly. As a result, the formation of an excellent full color image is hardly made, and deformation or curling of the thermal transfer image receiving sheet is much more strikingly brought about.
In addition, in the case of using the conventional thermal transfer image receiving sheets, there are such problems that the obtained printed materials are difficultly folded when they are intended to be folded or filed; they cannot be thinly folded even if the folding is possible; or they become bulky when filed, so that they are hardly applied to the ordinary office uses. Moreover, because of high cost and lacking of ordinary paper-like texture, they are unsuitable for ordinary office supplies.
In other conventional image receiving sheets in which the above-mentioned various substrate sheets are used and a dye receptor layer made of a thermal plastic resin such as a polyester resin, a vinyl chloride resin and a vinyl chloride/vinyl acetate copolymer resin is provided thereon, the dye receptor layer is easily peeled off due to the heat of the thermal head during the thermal transferring procedure or due to the adhesive tape.
For the formation of a sharp image, a sufficient whiteness of the dye receptor layer is necessary. However, when a large amount of a white pigment is introduced into the dye receptor layer for that purpose, deposition properties of the dye are decreased. Further, for obtaining an image of high resolution free from color dropout, decoloring, etc., the image receiving sheet is required to have sufficient cushioning properties so as to bring the dye receptor layer into good contact with the thermal head.
Such cushioning properties are generally obtained by forming an intermediate layer made of a resin having high cushioning properties between the substrate sheet and the receptor layer.
A most effective layer as the intermediate layer is a layer containing bubbles. In this case, however, when an image is formed by the thermal head, the bubbles-contained in the intermediate layer are expanded again owing to the heat of the thermal head to make the surface of the receptor layer depressed and protruded or to break through the receptor layer, whereby the receptor layer becomes defective to give an adverse effect to the resulting image.
By providing the intermediate layer, the cushioning properties of the receptor layer can be improved, but the physical strength thereof is lowered. For example, if writing with a pencil or the like is intended to be made on the receptor layer before or after the image formation, a lead of the pencil scratches and writing is difficult because of low strength of the receptor layer. Otherwise, if the writing is compulsively made, the receptor layer is peeled off. In the case of using the ordinary paper such as a PPC paper as the substrate sheet of the image receiving sheet as described before, there is brought about such a problem that unevenness occur on the surface of the dye receptor layer correspondingly to the roughness of the surface of the paper substrate. For solving this problem, a transfer method in which the dye receptor layer is transferred onto the surface of the paper is known. In this method, a receptor layer-transfer film having a dye receptor layer and an adhesive layer laminated on a surface of a substrate film having high releasability is employed.
However, since the adhesive layer of the conventional receptor layer transfer films uses a heat-sensitive thermoplastic resin, the transference of the receptor layer needs application of heat, so that it is difficult to conduct high-speed transference. Further, in the case of using a coarse substrate sheet (e.g., paper) as the substrate sheet, adhesion strength thereof is insufficient in the high-speed transference. Moreover, the resulting image receiving sheet does not have satisfactory cushioning properties.
Among the thermal transfer image receiving sheets used in the above-mentioned thermal transfer methods, those having a dye receptor layer made of a thermoplastic resin on the surface of the substrate sheet require that an image of a dye is provided on the dye receptor layer. Therefore, a sensor for discriminating between a front surface and a back surface of the image receiving sheet is fitted to the thermal transfer device, and any one of the front and back surfaces of the image receiving sheet is provided with a detection mark capable of being detected by the sensor.
The detection of the front and back surfaces is made by a conventional optical means, so that on the image receiving sheet is formed a black or black-like detection mark having a reflectance largely different from that of the image receiving sheet. Accordingly, such detection mark exists on the image-formed surface, and thereby an appearance of the obtained image becomes bad.
Of course, the detection mark may be provided on the back surface of the image receiving sheet, but in this case, the detection mark can be seen through from the front surface, resulting in bad appearance of the obtained image. Moreover, in the case of forming the dye receptor layer on each surface side of the image receiving sheet, the same problem as described above still remains.
Formation of various information such as a photograph of face in the above thermal transfer methods is carried out by deposition of the dye within the card substrate, so that thus formed various information shows high smoothness, alter-preventing properties and forgery-preventing properties. However, since the protective layer can be removed with a solvent, an acid, a base, etc., alteration or forging of photographs and other information is not completely prevented.
It is an object of the present invention is to solve the above-mentioned various problems accompanied by the prior arts, and to provide a thermal transfer image receiving sheet free from waving and curling even when the receptor layer is thickened and not producing any paper powder.
It is another object of the invention to provide a thermal transfer image receiving sheet capable of forming a dye image of high quality even in the case where high gradation and large difference in the density are required for the image.
It is a further object of the invention to provide a thermal transfer image receiving sheet available at a low cost, which can be easily folded and filed and has ordinary paper-like texture.
It is a still further object of the invention to provide a thermal transfer image receiving sheet excellent in smoothness, strength, cushioning properties and writing properties of the dye receptor layer and capable of forming an image of high density and high resolution.
It is a still further object of the invention to provide a thermal transfer image receiving sheet excellent in adhesion properties, whiteness, cushioning properties, etc.
It is a still further object of the invention to provide a thermal transfer image receiving sheet whose front and back surface sides can be easily discriminated in a printer and which can give an image of good appearance.
It is a still further object of the invention to provide a thermal transfer image receiving sheet capable of forming an image much more improved in alter-preventing properties and forgery-preventing properties.
A first embodiment of the invention is a thermal transfer image receiving sheet comprising a substrate sheet, an intermediate layer provided on at least one side surface of the substrate sheet and a dye receptor layer provided on the surface of the intermediate layer, wherein the substrate sheet is a pulp paper, the intermediate layer is formed from an organic solvent solution of a resin, and the dye receptor layer is formed from an aqueous resin liquid of a hydrophobic resin.
By the first embodiment, a thermal transfer image receiving sheet reduced in occurrence of curling caused by moisture variation can be obtained.
A second embodiment of the invention is a thermal transfer image receiving sheet comprising a substrate sheet and a dye receptor layer provided on at least one side surface of the substrate sheet, wherein at least one of the substrate sheet and the dye receptor layer contains a heat-absorbing material which absorbs heat at a temperature in the range of 80 to 200xc2x0 C.
By the second embodiment, the receptor layer is prevented from occurrence of depressed and protruded portions and the image receiving sheet can be prevented from deformation and curling, whereby a full color image of high quality can be formed.
A third embodiment of the invention is a thermal transfer image receiving sheet comprising a substrate sheet and a dye receptor layer provided on at least one side surface of the substrate sheet, wherein the substrate sheet is a paper substrate sheet having a basis weight in the range of 60 to 120 g/m2.
By the third embodiment, a thermal transfer image receiving sheet which can be easily folded and filed and is excellent in the ordinary paper-like texture can be obtained at a low cost.
A fourth embodiment of the invention is a thermal transfer image receiving sheet comprising a substrate sheet and a dye receptor layer provided on at least one side surface of the substrate sheet, wherein the substrate sheet is either a pulp paper impregnated with an aqueous resin or a pulp paper coated with an aqueous resin.
By the fourth embodiment, the substrate sheet of the thermal transfer image receiving sheet can be enhanced in the water retention characteristics to restrain releasing and absorption of water content from the substrate sheet, and the hydrophobic dye receptor layer can be made thin, so that curling caused by the environmental moisture variation and occurrence of paper powder can be restrained.
A fifth embodiment of the invention is a thermal transfer image receiving sheet comprising a substrate sheet, an intermediate layer provided on at least one side surface of the substrate sheet and a dye receptor layer provided on the surface of the intermediate layer, wherein the intermediate layer is formed from either an acrylic resin or a resin at least a part of which is crosslinked. This fifth embodiment also includes a thermal transfer image receiving sheet comprising a substrate sheet, a bubble-containing layer provided on at least one side surface of the substrate sheet, an intermediate layer provided on the surface of the bubble-containing layer and a dye receptor layer provided on the surface of the intermediate layer.
By the fifth embodiment, a thermal transfer image receiving sheet which is excellent in smoothness, strength, cushioning properties and writing properties of the dye receptor layer and capable of forming an image of high density and high resolution can be obtained.
A sixth embodiment of the invention is a thermal transfer image receiving sheet comprising a substrate sheet, an intermediate layer provided on at least one side surface of the substrate sheet and a dye receptor layer provided on the surface of the intermediate layer, wherein the intermediate layer is formed from a chlorinated polypropylene resin.
By the sixth embodiment, a thermal transfer image receiving sheet excellent in adhesion properties and cushioning properties can be obtained.
A seventh embodiment of the invention is a thermal transfer image receiving sheet comprising a substrate sheet, an intermediate layer provided on at least one side surface of the substrate sheet and a dye receptor layer provided on the surface of the intermediate layer, wherein the intermediate layer is formed from such a resin as to have a glass transition temperature in the range of xe2x88x9280 to 20xc2x0 C.
By the seventh embodiment, a thermal transfer image receiving sheet excellent in cushioning properties can be obtained.
A eighth embodiment of the invention is a thermal transfer image receiving sheet comprising a substrate sheet and a dye receptor layer provided on at least one side surface of the substrate sheet, wherein at least one side surface of the image receiving sheet has either a detection mark undistinguishable with the naked eye or an inconspicuous detection mark.
By the eighth embodiment, a thermal transfer image receiving sheet whose front and back surfaces can be easily discriminated in a printer and which can form an image of good appearance can be obtained.
A ninth embodiment of the invention is a thermal transfer image receiving sheet comprising a substrate sheet and a transparent dye receptor layer provided on at least one side surface of the substrate sheet, wherein an optional pattern is provided between the substrate sheet and the transparent dye receptor layer.
By the ninth embodiment, the pattern forms a background of the image, and accordingly, if a false photograph of face is attached thereto, the attached false photograph hides the pattern, whereby altering or forging becomes apparent. Otherwise, if the image is intended to be removed with special chemicals, the pattern behind the image is simultaneously eliminated, and an accurate recovery of the pattern is difficult.